In color image processing applications, it is useful to understand how humans perceive colors. By understanding the human visual system and its sensitivity to certain colors, one can more effectively create and manipulate images to create a desired visual effect. This assertion is particularly true in image processing applications that intentionally alter an image to perform a desired function, like hiding information in an image or compressing an image. In digital watermarking, for example, one objective is to encode auxiliary information into a signal, such as an image or video sequence, so that the auxiliary information is substantially imperceptible to humans in an output form of the signal. Similarly, image compression applications seek to decrease the amount of data required to represent an image without introducing noticeable artifacts.
A useful tool in watermark embedding and reading is perceptual analysis. Perceptual analysis refers generally to techniques for evaluating signal properties based on the extent to which those properties are (or are likely to be) perceptible to humans (e.g., listeners or viewers of the media content). A watermark embedder can take advantage of a Human Visual System (HVS) model to determine where to place an image watermark and how to control the intensity of the watermark so that chances of accurately recovering the watermark are enhanced, resistance to tampering is increased, and perceptibility of the watermark is reduced. Similarly, audio watermark embedder can take advantage of a Human Auditory System model to determine how to encode an audio watermark in an audio signal to reduce audibility. Such perceptual analysis can play an integral role in gain control because it helps indicate how watermark embedding can be adjusted relative to the impact on the perceptibility of the mark. Perceptual analysis can also play an integral role in locating the watermark in a host signal. For example, one might design the embedder to hide a watermark in portions of a host signal that are more likely to mask the mark from human perception.
The disclosure relates to selective color digital watermark embedding of images.
Digital watermarking is a process for modifying physical or electronic media to embed a machine-readable code into the media. The media may be modified such that the embedded code is imperceptible or nearly imperceptible to the user, yet may be detected through an automated detection process. Most commonly, digital watermarking is applied to media signals such as image signals, audio signals, and video signals. However, it may also be applied to other types of media objects, including documents (e.g., through line, word or character shifting), software, multi-dimensional graphics models, and surface textures of objects.
Digital watermarking systems typically have two primary components: an encoder that embeds the watermark in a host media signal, and a decoder that detects and reads the embedded watermark from a signal suspected of containing a watermark (a suspect signal). The encoder embeds a watermark by subtly altering the host media signal. The physical manifestation of altered host media most commonly takes the form of altered signal values, such as slightly changed pixel values, image luminance, color or color values, picture colors, altered DCT or transform domain coefficients, etc. The reading component analyzes a suspect signal to detect whether a watermark is present. In applications where the watermark encodes information, the reader extracts this information from the detected watermark.
Several particular watermarking techniques have been developed. The reader is presumed to be familiar with the literature in this field. Particular techniques for embedding and detecting imperceptible watermarks in media signals are detailed, e.g., in assignee's above-cited patent documents.
A watermark encodes auxiliary information in an image by modifying attributes of image samples in the image. As part of the encoding process, the encoder computes a change in an image attribute, such as luminance, to an image sample to encode auxiliary information in the image. The encoder changes color values of the image sample to effect the change in luminance with minimized impact on visibility.
One aspect of the disclosure provides a method of encoding auxiliary information in an image. The method includes computing a change in an attribute of an image sample to encode auxiliary information in the image; and changing color values of the image sample to effect the change in the image sample attribute. The changes to color values are determined based at least in part on both: i) visibility of the changes, and ii) anticipated watermark detection.
Another aspect of the disclosure is a method of decoding auxiliary information encoded in an image. The method includes receiving optical scan data corresponding to the image. The scan data includes first data corresponding to a first color channel, second data corresponding to a second color channel and third data corresponding to a third color channel. The method further includes weighting the first data, the second data and the third data according to at least the following two factors: i) a color direction anticipated to correspond with the embedding; and ii) anticipated image distortion introduced to the first data, second data or third data through optical scanning or signal processing. The method then includes determining from the weighted first data, second data and third data, changes in an image sample attribute, wherein the auxiliary information is conveyed through the changes.
According to still another aspect, a watermark embedding method includes: i) receiving an image including a generally white region; ii) determining a color change to embed a portion of a watermark signal in the white region in terms of a yellow channel; and iii) offsetting the yellow channel change in the white region with color changes in the cyan and magenta channels of the white region.
Further features of the technology will become even more apparent with reference to the following detailed description and accompanying drawings.